The large band-gap of diamond means that single photons do not have enough energy to promote electrons across the potential barrier; photoionisation must therefore occur through non-linear processes involving the simultaneous absorption of multiple photons and tunnelling ionisation. This process is often modelled for many materials using the Keldysh equation which assumes a direct gap and linear dispersion [1]}. However curvature of the bands leads to differing photoionisation such that for materials with differing dispersion behaviour, the Keldysh approach is inaccurate [2]}. A Monte Carlo approach to photoionisation for the diamond band-structure reveals that degeneracy in the valence bands at the gamma point leads to a nonperturbative quantum interference causing higher harmonic generation within the polarisation density of the crystal, significantly increasing the photoionisation rate at momentum states away from the centre of the Brillioun zone [3]}. Here we find that by making use of points of high symmetry, a hybrid of the Monte Carlo approach and the Keldysh approach leads to an accurate representation of photoionisation but with much less computational expense than a full Monte Carlo simulation.
